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CSU CHEM 113 - Exam 3 Study Guide

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Chem 113 1st EditionExam # 3 Study Guide Lectures: 18-26Lecture 18 (March 4)What does the reaction quotient, Q, represent? What is Q at equilibrium?Q represents the ratio of the product concentration to the reactant concentration at any point in a reaction; is indicates how close the reaction is to equilibrium and in what direction the reaction will go to reach equilibrium. It is expressed by the following equation: Q=[C ]c[D]d[ A ]a[B]b.Q is equivalent to K at equilibrium.What is the law of mass action?The law of mass action states that at a given temperature, a chemical system reaches a state which a state in which a particular ratio of reactant and product concentrations has a constant value.How do you determine the direction of a reaction?If Q<K, the reactants must decrease and the products increase; reactants products until equilibrium is reached. If Q>K, the reactants must increase and the products decrease; productreactants until equilibrium is reached. If Q=K, the system is at equilibrium and no further net change takes place.Lecture 19 (March 6)Explain Le Châtelier’s Principle.When a chemical system at equilibrium is disturbed, i.e. if the change in conditions forces it temporarily out of equilibrium, the system reattains equilibrium by undergoing a net reactionthat reduces the effect of the disturbance. A shift to the left is a net reaction from product to reactant. A shift to the right is a net reaction from reactant to product.How do changes in concentration effect Le Châtelier’s Principle?If the concentration of reactants increases, the equilibrium position shifts to the left. If the concentration of products increases, the equilibrium position shifts to the right. Only substanceswhose concentrations appear in the expression for Q have an effect on the equilibrium position.A change in concentration has no effect on the value of K (at equilibrium).How do changes in pressure/volume effect Le Châtelier’s Principle?Changes in pressure affect equilibrium systems containing gaseous components by changing theconcentration of a gaseous component. Adding an inert gas has no effect on the equilibrium position as long as the volume doesn’t change, because all concentrations and partial pressures remain unchanged. Changing the volume of the reaction vessel will cause equilibrium to shift if Δngas≠0.How do changes in temperature effect Le Châtelier’s Principle?Heat is considered a component of the system in order to determine the effect of a change in equilibrium of a system. Heat is a product in an exothermic reaction and a reactant in an endothermic reaction. An increase in temperature adds heat, which favors the endothermic reaction. A decrease in temperature removes heat, which favors the exothermic reaction.Lecture 20 (March 9)What are the major steps when solving equilibrium problems?Write the balanced equation. Write the expression for the reaction quotient Q or K. Convert all amounts into the correct units (M or atm). Set up a reaction table (Reaction, Initial quantities of reactants and products, changes, equilibrium quantities). Check the sign of x to determine the change in concentration (or pressure).How do you justify the assumption that x is negligible when solving the equilibrium quantities?You justify the assumption by proving that x has <5% error; if it doesn’t, solve for x using the quadratic equation.Lecture 21 (March 11)Under what circumstances can we assume x of [A]reacting can be neglected?Kc is relatively small and [A]init is relatively large. If [ A ]initKc>400, the assumption is justified; neglecting x introduces an error <5%. If [ A ]initKc<400, the assumption is not justified; neglecting x introduces an error >5%How can the change in Gibbs Energy be determined under nonstandard conditions?Using the following equation: ΔG = ΔG° + RT lnQWhat is the difference between ΔG° in a gas phase compared to a solution?The ΔG° in a gas phase relates to the Kp value whereas the ΔG° in the solution relates to the Kc value.Lecture 22 (March 13)What is the equation that expresses the relationship between ΔG, Q, and K?ΔG=RTlnQK=RTlnQ−RTlnKIf Q and K are very different, ΔG has a very large (positive or negative) value, meaning that the reaction releases or absorbs a large amount of free energy. If Q and K are nearly the same, ΔG has a very small (positive or negative) value and the reaction absorbs or releases very little free energy.How does ΔG and equilibrium help determine the direction of a reaction?A reaction proceeds spontaneously to the right if Q<K; lnQK<0 and ΔG<0. A reaction proceeds spontaneously to the left if Q>K; lnQK>0 and ΔG>0. A reaction is at equilibrium if Q=K; QK=1 so lnQK=0 and ΔG=0.What is the relationship between ΔG and the equilibrium constant (K)?Due to their logarithmic relationship, a small change in ΔG causes a large change in K. As ΔG becomes more positive, the value of K becomes smaller. As ΔG becomes more negative, K becomes larger.What is the only factor that affects the value of K for an equilibrium system? How so?Temperature; for an endothermic reaction, an increase in temperature will cause the value of K to increase. For an exothermic reaction, an increase in temperature will cause K to decrease.What is the van’t Hoff equation?lnK2K1=−∆ H °rxnR(1T2−1T2)Lecture 23 (March 23)What are Bronsted-Lowry Acids and Bases?A Bronsted-Lowry Acid is a proton (H+) donor in a reaction and it must contain H in its formula. ABronsted-Lowry base is a proton (H+) acceptor in a reaction and it must contain a lone pair of electrons to bond to the H+.What is the difference between strong and weak acids? Give examples of both.A strong acid dissociates completely into ions in water and has a Kc value much larger than 1. Strong acids include hydrohalic acids and oxoacids in which the number of O atoms exceeds the number of ionizable protons by 2 or more. A weak acid dissociates slightly to form ions in water and has a Kc value much less than 1. Weak acids include HF, acids in which H isn’t bonded to O or a halogen, oxoacids in which the number of O atoms equals or exceeds the number of ionizable protons by one, and carboxylic acids.Lecture 24 (March 25)What are some examples of strong bases? How can you tell they are a strong base?Strong bases are usually water soluble compounds containing O2- or OH- ions. Cations are usually those of the most active metals (i.e. those in groups 1 and 2 on the


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CSU CHEM 113 - Exam 3 Study Guide

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